--- a/mercurial/util.py	Sun Feb 10 13:19:24 2008 +0100
+++ b/mercurial/util.py	Mon Feb 11 16:13:43 2008 -0600
@@ -459,6 +459,8 @@
             return
         try:
             pat = '(?:%s)' % '|'.join([regex(k, p, tail) for (k, p) in pats])
+            if len(pat) > 20000:
+                raise OverflowError()
             return re.compile(pat).match
         except OverflowError:
             # We're using a Python with a tiny regex engine and we